Oculogenital disease induced by the sexually transmitted pathogen Chlamydia trachomatis manifests as a range of acute and chronic sequelae that represent a significant source of morbidity in humans. Chlamydiae are obligate intracellular pathogens that develop within a membrane-bound vacuole termed an inclusion. Although inclusions represent a privileged niche, sequestered chlamydiae possess the ability to modulate host-cell functions in order to create and maintain a permissive growth environment. Chlamydiae express a type III secretion system that, in other gram-negative pathogens, is a secretion mechanism strongly associated with pathogenesis. I have identified two chlamydial type III substrates and propose to identify specific interactions of these substrates with host molecules and delineate the consequences of those interactions. Using yeast two-hybrid assays, I propose to identify specific interactions between chlamydial effectors and host proteins. These elucidated in vitro interactions will be confirmed in chlamydial infections using a series of crosslinking and coprecipitation assays in combination with mass spectroscopy. The consequences of these interactions will be investigated in vitro by ectopic expression of effector proteins followed by biochemical confirmation of detected effects in infected cells. The chlamydial type III secretion system represents an attractive, yet unexplored, mechanism to achieve modulation of host cell activities. By specifically elucidating which host pathways are targeted I will gain insight into i) molecular mechanisms employed by Chlamydia to sculpt their intracellular environment and ii) what host cellular processes are important to chlamydial survival.